| Low density polyethylene is widely used in high voltage power cable insulation due to its excellent dielectric and mechanical properties. With the construction and operation of UHV and HVDC System in our country, the aging problem of power cable caused by space charge is more and more serious nowadays. In order to ensure the safe operation of power cables, it is essential to find a way to reduce the space charge effect of polyethylene. In this paper, low-density polyethylene (LDPE)/montmorillonite (MMT) nano-composites were prepared by a melt intercalation process; the space charge properties of LDPE/MMT under different direct current (DC) electric field strength were experimentally and theoretically analyzed. The main contents were as follows:â‘ The intercalation and dispersion behavior of nano-montmorillonite in polyethylene were investigated by means of X-ray diffractometry(XRD) and transmission electron microscope(TEM).The thermal properties, impact properties, and dielectric strengths were analyzed by methods of Thermo-gravimetric Analysis(TGA) and Izod impact test and electrical breakdown test. Experiment results showed that the thermal properties, impact strength and dielectric strength of low density polyethylene were improved by Nano-MMT.â‘¡Cooling was an important part of the production process of Polyethylene cable, therefore, the three types of LDPE specimens used for space charges experiments were obtained by cooling in ice water, air, and silicone oil. The dissipation characteristics of space charges were investigated based on the space charges testing system of pulse electro-acoustic (PEA) approach. According to further analysis of the test result of differential scanning calorimetry (DSC), the influence of cooling medium on space charge transport property of LDPE was investigated. The LDPE cooled in silicon oil has the highest threshold stresses,the highest apparent mobility of charge and the greatest rate of dissipation.â‘¢Thermal aging was a major cause of damage for polyethylene cable insulation; Therefore, the accumulation and dissipation characteristics of space charges of LDPE under different thermal condition and aging were investigated. According to further analysis of the test result of Fourier translation infrared spectroscopy (FT-IR), the influence of thermal aging condition on space charge of PE was analyzed. The results showed that thermal aging made LDPE accumulate more space charge, and decreasing the dissipation rate of space charge after the removal of the applied voltage. The influence of space charge accumulation and dissipation of PE under hydrothermal condition was greater than that under dry-thermal condition.â‘£The accumulation and dissipation characteristics of space charges of LDPE/MMT were investigated based on the space charges testing system of pulse electro-acoustic (PEA) approach. According to the test result of differential scanning calorimetry (DSC) and dielectric frequency spectra, including space charge trap theory, the inhibition mechanism of added MMT nano-particles to restrain space charge in LDPE was explained. Experiments and analysis results showed that the addition of MMT nano-particles restrained the accumulation of space charge of LDPE under DC voltage, improved the rate of dissipation of space charge of LDPE after the removal of the applied voltage, and reduced the influence of space charge of cable for DC high voltage transmission. |
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